The present invention generally relates to semiconductor processing and, more particularly, to systems and methods for measuring and/or imaging features, such as lines and spaces, including those having reentrant profiles.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these higher device densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish higher device densities, smaller and smaller features sizes are required. These may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry of corners and edges of various features.
High resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist. The film is selectively exposed with radiation (such as optical light, x-rays, or an electron beam) through an intervening master template, the mask, forming a particular pattern. Exposed areas of the coating become either more or less soluble than the unexposed areas (depending on the type of coating) in a particular solvent developer. The more soluble areas are removed with the developer in a developing step. The less soluble areas remain on the silicon wafer forming a patterned coating. The pattern corresponds to the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
At various stages in forming the patterned resist coating and processing the silicon wafer, it is desirable to measure critical dimensions resulting from the lithographic process. Critical dimensions include the size of features in the wafer or patterned resist such as line widths, line spacing, and contact dimensions. Due to the extremely fine patterns involved, Scanning Electron Microscopy (SEM) is often employed to analyze critical dimensions.
In SEM, an electron beam is scanned across the sample. The beam interacts with the sample to produce measurable responses that vary with position over the course of a scan. Measurable responses include backscattering of electrons and production of secondary electrons, auger electrons, X-rays and cathodoluminescence. Secondary electrons are the most useful of the measurable responses in accessing surface topography and are the responses most often measured in critical dimension analysis. A secondary electron detector is used to measure the variation in secondary electron intensity over the course of a scan. An image formed of secondary electron intensity measurements is comparable to a black and white picture of the surface taken from the perspective of the electron beam with illumination coming from the position of the secondary electron detector.
While such images are useful is critical dimension analysis, they have some important limitations. For example, in certain fabrication processes, resist and/or etched features have cross-sectional profiles that are reentrant. By xe2x80x9creentrant profile,xe2x80x9d it is meant that feature sidewalls taper inwardly towards the base of the feature. For an elongated feature, such as a line or space, a reentrant profile may result in an elongated trench (e.g., having a trapezoidal cross section) positioned along the juncture of the feature and the substrate surface adjacent the feature. While reentrant profiles may be desirable in certain circumstances, the reentrant features may cause a shadowing effect during subsequent deposition. As a result of the shadowing effect by the upper portion of the feature, an elongated void may be formed during the deposition at the base of the reentrant feature where it contacts the substrate. The void, if undetected, may have serious consequences for subsequent processing steps and may result in defects that compromise the operation of the resulting semiconductor device. Conventional SEM systems for measuring critical dimensions of wafers often fail to detect reentrant profiles of lines and/or spaces.
It is desirable to have systems and methods that facilitate measuring and/or imaging a feature, such as a line and/or trench, having a reentrant profile.
The present invention provides a system and method that facilitates measuring and imaging topographical features of a substrate, including lines and trenches having reentrant profiles. One aspect of the invention provides an electron microscope that simultaneously scans a substrate with two or more electron beams that are directed against the substrate with substantially differing angles of incidence. Secondary electrons resulting from the interaction of the substrate with the beams are detected by one or more secondary electron detectors. Each secondary electron detector may simultaneously receive secondary electrons resulting from the interaction of the substrate with two or more electron beams. In another of its aspects, the invention provides methods of analysis that permit the interpretation of such data to analyze critical dimensions and form images of the substrate. Critical dimensions that may be determined include feature heights and reentrant profile shapes. The topographical information provided is more complete than that of conventional SEM imaging and is obtained more rapidly than would be possible using multiple scans of a single electron beam.
One aspect of the invention provides a scanning electron microscope including an electron beam source, electromagnetic elements configured to simultaneously direct with substantially differing angles of incidence a first and a second electron beam against a substrate, and a first secondary electron detector configured to detect at least secondary electrons resulting from an interaction of the substrate with the first electron beam
Another aspect of the invention provides a system for measuring a characteristic of a reentrant topographical feature of a substrate including means for simultaneously directing two or more electron beams at the substrate surface wherein two of the beams are directed at angles differing by at least about 10 degrees, and means for detecting secondary electrons produced by the interaction of the substrate with the electron beams.
A further aspect of the invention provides a method for assessing a characteristic of a feature of a substrate surface, the method including scanning the substrate simultaneously employing first and second electron beams directed against the substrate, the first and second electron beams having angles of incidence that differ by at least about 10 degrees, detecting secondary electrons produced by interaction of the first and second electron beams with the substrate to generate secondary electron data, and analyzing the secondary electron data to assess the characteristic of the feature of the substrate surface.
A further aspect of the invention provides a scanning electron microscope system including an electron beam source, electromagnetic elements for simultaneously directing first and second electron beams derived from the electron beam source against a substrate, a first secondary electron detector for simultaneously detecting secondary electrons resulting from the interaction of the substrate with the first electron beam and secondary electrons resulting from the interaction of the substrate with the second electron beam and for sending data relating to the secondary electrons detected, and a processor for receiving and analyzing the data from the secondary electron detector.
A further aspect of the invention provides a system for measuring a characteristic of a reentrant topographical feature of a substrate, including means for simultaneously directing two or more electron beams at the substrate, the beams having angles differing by at least about 10 degrees, means for detecting secondary electrons resulting from the interaction of the substrate with the electron beams and producing secondary electron data, and means for analyzing secondary electron data, wherein the secondary electrons resulting from the interaction of the substrate with two or more of the electron beams are detected together.
A further aspect of the invention provides a method for assessing a characteristic of a feature of a substrate surface, the method including scanning the substrate simultaneously employing first and second electron beams directed against the substrate, detecting secondary electrons resulting from an interaction of the first electron beam with the substrate together with secondary electrons resulting from an interaction of the second electron beam with the substrate to generate secondary electron data, and analyzing the secondary electron data to assess the characteristic of the feature of the substrate surface.
The invention extends to features hereinafter fully described and features particularly pointed out in the claims. The following detailed description and the annexed drawings set forth in detail certain illustrative examples of the invention. These examples are indicative of but a few of the various ways in which the principles of the invention may be employed. Other ways in which the principles of the invention may be employed and other objects, advantages and novel features of the invention will be apparent from the detailed description of the invention when consider in conjunction with the drawings.